U.S. Pat. Nos. 3,806,312; 3,934,970; 3,947,242; and 3,994,711 disclose furnaces including roller conveyors for heating glass sheets while conveyed in a generally horizontal orientation through an associated heating chamber. Convection between both the top and bottom surfaces of the conveyed glass sheets and the gas within the heating chamber performs a certain extent of the heating. Conduction between heated rolls of the conveyor and the bottom surface of each conveyed glass sheet also provides some of the heating. In addition, the preferred construction of these furnaces utilizes electric resistance elements both above and below the conveyor rolls such that both the top and bottom surfaces are also heated by radiation. Of course, the rolls shield the bottom surface of each glass sheet to some degree from the resistance elements located below the conveyor and thereby decrease the radiant energy that is transferred to the glass.
It is important to maintain glass sheets generally planar during heating on roller conveyors regardless of whether the final product is to be flat glass for architectural use or bent glass such as used with vehicles. Due to conduction of heat from the rolls, the bottom surface of the glass sheet heats more rapidly causing the glass sheet to be convex at its bottom surface and causing the central area which remains in contact with the rolls to overheat which warps the glass sheet and also results in roll print at the central area. Flat glass is usually tempered or is first coated with a metal film and then tempered by rapid cooling of the heated glass sheet which cannot deviate too much from a uniform temperature if the final product is to be flat. Depending upon the type of bending equipment utilized, it can be difficult to provide an accurate control of the shape of bent glass if the heated glass sheet deviates from a uniform temperature by too great an extent.
Heating of glass sheets to a desired temperature for processing is most preferably performed as rapidly as possible to keep the furnace short to lower heat losses and so as to provide heating with as little distortion as can be achieved.
U.S. Pat. Nos. 3,778,244; 4,202,681; and 4,222,763 disclose bending stations including gas jet pumps utilized in association with roller conveyor type furnaces within which glass sheets are heated. These gas jet pumps are supplied pressurized gas through a nozzle opening so as to effect a primary gas flow which induces a secondary gas flow in order to support a heated glass sheet in preparation for bending or to provide the bending against a curved surface located above the roller conveyor.
U.S. Pat. Nos. 4,059,426 and 4,059,427 disclose glass sheet heating furnaces of the gas support type wherein gas jet pumps pressurize a plenum chamber below a support bed having passages through which the pressurized gas escapes to support a glass sheet above the bed for conveyance therealong and heating within the furnace heating chamber. Pressurized gas fed to the gas jet pumps from outside of the heating chamber is throttled to provide a primary gas flow along a curved surface so as to thereby induce a secondary gas flow of a greatly amplified extent for pressurizing the chamber that feeds the gas through the passages of the support bed on which the glass sheets are conveyed. Also, the U.S. Pat. No. 4,059,426 patent additionally discloses such gas jet pumps positioned above the gas support bed so as to direct heated gas flow downwardly against the top surfaces of glass sheets conveyed on the gas support bed. Electrical resistance heating of the compressed gas supplied to such jet pumps provides the energy input to maintain the heated condition of the gas within this furnace.
U.S. Pat. No. 4,204,854 discloses gas jet pumps utilized with a glass sheet heating furnace of the roller conveyor type so as to blow heated glass sheets upwardly off of the conveyor rolls against a holder positioned above the conveyor in preparation for depositing of the glass sheet on a mold for bending.
U.S. Pat. No. 4,282,026 discloses a glass sheet bending system in which a heated glass sheet is lifted upwardly from a horizontal conveyor and supported by a differential gas pressure against a downwardly facing surface of a holder in preparation for bending. Termination of the differential gas pressure releases the heated glass sheet so as to drop downwardly onto a mold of a curved shape for bending. An actuator moves the mold to a quench unit in a time to control the extent of bending before tempering of the glass sheet.